Fuel cells are devices that convert fuel and oxidant to electrical energy. Electrochemical cells generally include an anode electrode and a cathode electrode separated by an electrolyte. A well known use of electrochemical cells is in a stack for a fuel cell that uses a proton exchange membrane (hereafter “PEM”) as the electrolyte. In such a cell, a reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. The hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
Most efficient fuel cells use pure hydrogen as the fuel and oxygen as the oxidant. Unfortunately, use of pure hydrogen has a number of know disadvantages, not the least of which is the relatively high cost, and storage considerations. Consequently, attempts have been made to operate fuel cells using other than pure hydrogen as the fuel.
For example, attempts have been made to use hydrogen-rich gas mixtures obtained from steam reforming methanol as a fuel cell feed. This may be particularly important for automotive applications, since a convenient source of hydrogen gas can be the steam reformation of methanol, since methanol can be stored more easily in a vehicle than hydrogen. However, it is known that methanol reformate gas can contain as much as 25% carbon dioxide (CO2) and up to 1% carbon monoxide (CO), and the catalytic performance of pure platinum can be significantly reduced by the presence of even 10 parts per million (ppm) of CO.
Therefore, successful use of reformed hydrogen fuel depends upon either decreasing the CO content of the fuel or development of CO-tolerant anode electrocatalysts, or both.
One approach to avoid the effects of CO on polymer electrolyte fuel cell (PEFC) performance is described in U.S. Pat. No. 6,245,14B1, wherein several methods for reducing CO concentrations by several additional fuel processing steps (prior to introduction of the fuel to the fuel cell stack) are outlined. All suffer from the drawback of substantially increasing the cost and complexity of the entire fuel cell system.
Another method for removing or lowering CO in a reformate fuel mixture is the oxidation of CO to CO2 at the anode by means of introducing air, typically 2% by volume, into the reformate hydrogen stream, as described in U.S. Pat. No. 4,910,099 (“air bleed method”). While this method is effective, it also introduces added complexity to the PEFC, and a loss of efficiency.
A further approach is to enhance the CO tolerance of the anode electrocatalyst in the PEFC. CO tolerance of Pt electrodes can be improved by alloying the electrocatalyst with a second element, preferably ruthenium (Ru) (see, for example, M. Iwase and S. Kawatsu, Electrochemical Society Proceedings, v. 95-23, p. 12; Proceedings of the First International Symposium on Proton Conducting Membrane Fuel Cells, S. Gottesfeld, et al., Eds., The Electrochemical Society, for a 1:1 atomic ratio alloy of Pt:Ru on a carbon support at Pt loading level of 0.4 mg/cm2, the fuel cell operating at 80° C. It is further known in the art (T. A. Zawodzinski, Jr, presented at Fuel Cells for Transportation, U.S. Department of Energy, National Laboratories, R &D Meeting, Jul. 22-23, 1997, Washington, D.C.) that a PEFC having a PtRu mass loading of 0.6 mg/cm2 operating at temperatures above 100° C. has been shown to be tolerant to 100 ppm CO. However, this method loses effectiveness at lower temperatures, and especially when lower loadings of the electrocatalyst are used.
In addition to CO tolerance at low temperatures, the use of CO tolerant electrocatalyst at a lower loading of electrocatalyst is needed. A low noble metal electrocatalyst loading would offer major advantages in cost, since noble metals comprise the majority of the cost in a typical noble metal based electrocatalyst system. Tungsten carbide is mentioned as a fuel cell catalyst component in U.S. Pat. No. 3,833,423. However, in this application, tungsten carbide is used as the sole ingredient in addition to covering it with other particles to provide autooxidation (degradation) of the electrocatalyst. The electrocatalytic activity for this system is very low.
WO 99/42213 discloses a catalyst comprising a support body comprised of a transition metal based electrically conductive ceramic, and at least one noble metal supported upon said support body. The transition metal based ceramic comprises a compound of at least one transition metal, the compound being selected from the group consisting of carbides, nitrides, borides, silicides and combinations thereof. In particular embodiments the ceramic may further include an oxide, oxycarbide or oxynitride therein. The noble metal may comprise a single metal, an alloy of metals, and one particularly preferred alloy comprises an alloy of alloy of platinum and molybdenum. Also disclosed as noble metals are Pt, Pd, Os, Ir, Ru, Ag, and Rh. The presence of an oxygen component, e.g., oxycarbide on the support diminishes the poisoning effects of CO poisoning on many noble metal catalysts.
Similarly, high surface area electrodes used in electrochemical energy storage devices comprising conductive transition metal nitrides, carbides and borides are disclosed in U.S. Pat. No. 5,680,292. The use of the pure materials are disclosed but not their composites with noble metals.
In an organic/air fuel cell an organic fuel such as methanol, formaldehyde, or formic acid is oxidized to carbon dioxide at an anode, while air or oxygen is reduced to water at a cathode. Fuel cells employing organic fuels are extremely attractive for both stationary and portable applications, in part, because of the high specific energy of the organic fuels, e.g., the specific energy of methanol is 6232 Wh/kg. One such fuel cell is a “direct oxidation” fuel cell in which the organic fuel is directly fed into the anode, where the fuel is oxidized. Thus the need for a reformer to convert the organic fuel into a hydrogen rich fuel gas is avoided resulting in considerable weight and volume savings for the fuel cell system.
Materials customarily used as anode electrocatalysts are pure metals or simple alloys (e.g., Pt, Pt/Ru, Pt/Ni) supported on high surface area carbon. For example, the state-of-the-art anode catalysts for hydrocarbon (e.g., direct methanol) fuel cells are based on platinum (Pt)-ruthenium (Ru) alloys. Heretofore, the best known catalyst was Pt50/Ru50 (numbers in subscript indicate atomic ratios). Gasteiger et al., J Phys. Chem, 98:617, 1994; Watanabe et al., J ElectroanaL Chem., 229:395, 1987. These known catalysts do not provide the required methanol oxidation to make them function effectively in fuel cells.
A need exists for fuel cell anode electrocatalysts that are tolerant to the high CO content of a CO-containing hydrogen rich gas mixture, thus minimizing the need for additional CO clean-up or abatement steps prior to the use of this fuel in a fuel cell stack. Fuel cell anode electrocatalysts containing a small amount of noble metal that render them less expensive than current anode electrocatalysts, at a comparable anode electrocatalyst loading, are also needed. A need also exists for improved catalysts that provide enhanced methanol oxidation in direct methanol fuel cells.